LED Measurement Series: LED Luminaire Reliability
Building Technologies Program
LED Measurement Series:
LED Luminaire Reliability
One of the most basic decision factors for light source selection is lifetime:
how long will it last? Long life is potentially a key advantage of LEDs over
other light sources. However, the lighting industry currently has very
limited direct experience with long-term performance and reliability of LED
luminaires. This document outlines the issues and provides suggestions
for understanding and interpreting LED product life claims.
Background
Lifetimes of traditional light sources (incandescent, fluorescent, and highintensity discharge lamps) are estimated through industry-standard lamp
rating procedures. Typically, a large, statistically-significant sample of lamps
is operated until 50% have failed; that point, in terms of operating hours,
defines “rated life” for that lamp. Based on years of experience with traditional
light sources, lighting experts can confidently use lamp life ratings, along with
known lumen depreciation curves, to design the lighting for a space, and to
determine re-lamping schedules and economic payback.
LED technology changes several aspects of this traditional approach:
1. LEDs usually do not fail abruptly like traditional light sources; instead
their light output slowly diminishes over time.
2. LEDs are often integrated permanently into the fixture, making their
replacement difficult or impossible.
3. LED light sources can have such long lives that life testing and acquiring
real application data on long-term reliability becomes problematic—new
versions of products are available before current ones can be fully tested.
4. LED light output and useful life are highly dependent on electrical and
thermal conditions that are determined by the luminaire and system design.
LED luminaires and replacement lamps available today often claim long life,
e.g., 50,000 hours or more, which exceeds the life ratings of nearly all other
light sources (except for some electrodeless sources). These claims are based on
the estimated lumen depreciation of the LED used in the product and often
do not account for other components or failure modes. Life claims by LED
luminaire manufacturers should take into account the whole system, not just
the LEDs. One of the key lessons learned from early market introduction
of compact fluorescent lamps is that long life claims need to be credible and
backed up with appropriate manufacturer warranties.1
1
US DOE. Compact Fluorescent Lighting in America: Lessons Learned on the Way to Market. 2006.
Light replacement carries real costs.
Photo Credit: University of Toronto
Terms
Lumen depreciation – the decrease
in lumen output that occurs as a lamp
is operated. For most light sources,
the lamp fails before significant lumen
depreciation occurs (an outdated exception is mercury vapor lamps). Because
LEDs can continue to operate at very
low light levels, LED life is usually
defined as lumen depreciation to a
particular point, such as 70% of
initial lumens.
Catastrophic failure – outright
failure of the lamp or luminaire, in
which the device stops working and
cannot be repaired.
L70 or L70 – shorthand for lumen
depreciation to 70% of initial lumen
output; stated conversely, it indicates
70% lumen maintenance. L50 would
be lumen depreciation of 50%. Different
light sources have different rates of lumen
depreciation. High-quality T8 fluorescent
lamps maintain 95% (L95) of initial
lumens at end of life.
B50 – another aspect of LED life
projection, used in conjunction with
the lumen depreciation (L) metric and
a target statistical confidence interval.
B50 indicates no more than 50% of
a sample of LED devices would be
expected to fail before a certain number
of operating hours. Failure means light
output drops below a target lumen
maintenance level (such as L70 or L50).
B10 would mean no more than 10% of
the sample fails within the given time.
LED Measurement Series: LED Luminaire Reliability
LED Device Lifetime
For white light LEDs providing general illumination, the definition of useful life is often given as the hours of operation
at which the LED’s light output has decreased to 70% of initial output (abbreviated as L70 or L70). The selection of
70% is based on vision research indicating that in general lighting applications, the “typical” human eye does not detect
the decrease in light until it exceeds 30%. LED manufacturers publish lumen depreciation curves based on testing of
their products, extrapolating lumen depreciation to the 70% level. Depending on the application, other depreciation
levels may be appropriate as end of life limits, such as L50 or L80.
Lifetime (Hours)
LED manufacturers make lumen maintenance projections based on extended in-house testing and statistical extrapolation,
accounting for the effects of drive current and LED junction temperature of the device in operation. For example, Philips
publishes curves shown in Figure 1. This
(B50, L70) lifetimes for InGaN Luxeon K2
graph shows the projected lifetimes (B50,
70,000
L70 means 50% of the products have at
least 70% lumen maintenance for the
60,000
projected number of operating hours) for
350mA
50,000
the K2 LED package. According to this
700mA
graph, 50% of a sample of K2 devices run
40,000
1A
at 350 mA would be expected to main30,000
tain at least 70% of initial lumens for
1.5A
60,000 hours, if the junction tempera20,000
ture is maintained at about 160C or
lower. If the device is run at a higher
10,000
current, e.g., 700 mA, Tj would have
0
to be kept lower (about 140C) to last
90
100
110
120
130
140
150
160
170
180
190
200
60,000 hours.
Junction temperature (C)
Figure 1. The curves show expected LED life based on drive current indicated by each colored line and target
At this time, there is not a standard
LED junction temperatures. Source: Philips Lumileds.
reporting format for LED lifetime or
lumen depreciation curves. A test procedure currently in development by the Illuminating Engineering Society of
North America (designated LM-80, IESNA Approved Method for Measuring Lumen Maintenance of LED Light
Sources) will provide a common procedure for making lumen maintenance measurements at the LED device, array,
or module levels.
LED Luminaire Lifetime
The LM-80 test procedure addresses only one factor in the life of an LED luminaire – lumen depreciation of the LED
device over the prescribed test period. When LEDs are installed in a luminaire or system, there are many additional
factors that can affect the rate of lumen depreciation or the likelihood of catastrophic failure. These include temperature extremes, humidity, moisture incursion, voltage or current fluctuations, failure of the driver or other electrical
components, damage or degradation of the encapsulant material covering the LEDs, damage to the wire bonds that
connect the LEDs to the fixture, and degradation of the phosphors.
LED performance is directly linked with other components in the luminaire, all critical for understanding product reliability. Current testing efforts and related research have provided some confidence in the reliability of some components.
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However, other characteristics of the technology,
interactions, and application
remain untested. Many
LED luminaires are newly
designed products, increasing the likelihood users will
experience unanticipated
problems relative to fixtures
that have been manufactured and refined for years.
As depicted in Figure 2,
Figure 2. Examples of multiple failure factors in electronic assemblies.
LED luminaires are
electronic assemblies involving several connection points, components and materials where degradation or failures can
take place, in spite of the use of high-quality, durable LEDs.
While the lifetime of an LED source is one important indicator of LED luminaire life, it would be misleading to rate
the entire LED luminaire based solely on the LED source. There is often a huge gap between the warranted life of a
product and the expected life of the LED source in that product. Further, reliability of fixtures that include replaceable
LED engines and replaceable components should be assessed differently than reliability of entirely integrated fixtures.
Results of LED Luminaire
Reliability Testing to Date
The DOE CALiPER program began
reliability testing on SSL luminaires
in August 2007. Some luminaires
maintain output levels over the
first 6,000 hours of operation
(7 of 26 products are producing
at least 95% of their initial output),
while others exhibit rapid lumen
depreciation within the first 1,000
to 2,000 hours, and some products
exhibit significant color shift over the
first 6,000 hours of operation. No generalizable patterns can be observed yet.
Figure 3. Interim results from CALiPER lumen depreciation testing. October 2009. Source: CALiPER Round 9
Summary Report.
Figure 3 summarizes lumen depreciation interim testing results for 26 SSL products. The lumen depreciation testing
is not completed for these samples, but these interim results already provide insight. The two white lines in the plot
are provided as reference curves: the horizontal white line indicates 70% of the initial output, and the descending
white curve represents a typical logarithmic decay that would reach L70 at 50,000 hours.
Research that Works!
LED Measurement Series: LED Luminaire Reliability
These 26 products cover a range of LED configurations, including task lamps,
replacement lamps, retrofit lamps, and outdoor area luminaires. At this point
of testing, and given the small sample size, one cannot draw conclusions about
the lumen depreciation performance of any particular category of products
(based on size or application). The CALiPER program will continue to collect
lumen depreciation and chromaticity maintenance data on a range of products,
and to make those testing results publicly available.
Key Points for LED Luminaire Reliability
In summary, LED luminaire life is not identical to estimated LED life. LED
luminaire life is also a function of the power supply, operating temperatures,
thermal management, materials, and electrical and material interfaces. Definitive lifetime ratings will not be possible until more experience is logged with
a wide range of LED luminaires in the field. In the meantime, how can LED
luminaire reliability be assessed? Some things to look for:
•
Use of high-quality LEDs from manufacturers who publish
reliability data.
•
Luminaire warranty offered by the manufacturer – should be at least
comparable to traditional luminaires used for the application under
consideration.
•
Luminaire photometric report, based on LM-79-08 test procedure,
from an independent testing laboratory.
•
Temperature data (for example, board, case, or solder joint temperature) for the LEDs when operated in the luminaire in the intended
application; and information about how the measured temperature
relates to expected life of the system.
•
Any test data available about longer term performance of the LED
luminaire, such as DOE CALiPER testing, manufacturer in-house
testing, or field tests conducted by DOE, utilities, or other parties.
A Strong Energy Portfolio
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Energy efficiency and clean,
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independence for America.
Working with a wide array of
state, community, industry, and
university partners, the U.S.
Department of Energy’s Office of
Energy Efficiency and Renewable
Energy invests in a diverse portfolio
of energy technologies.
For more information contact:
EERE Information Center
1-877-EERE-INF
(1-877-337-3463)
www.eere.energy.gov
For Program Information
on the Web:
www.ssl.energy.gov
DOE sponsors a comprehensive
program of SSL research,
development, and commercialization.
For Program Information:
Robert Lingard
Pacific Northwest National
Laboratory
Phone: (503) 417-7542
E-mail: robert.lingard@pnl.gov
PNNL-SA-61137
October 2009
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